An axial-piston pump generally comprises a cylinder drum provided with a plurality of angularly equispaced cylinder bores (hereinafter cylinders) in which respective pistons are reciprocatable. The pistons bear upon an inclined disk or swash plate or a drive flange, depending upon the type of axial-piston machine and, when the plane of this member lies at an angle to the axis of the drum, the machine can operate either as a pump or as a motor. In the first case, the cylinder drum is rotated so that the pistons reciprocate within the cylinders and alternately draw fluid into the cylinder bores and express fluid therefrom.
When the machine operates as a motor, fluid is forced under pressure into the cylinder bores, causing the cylinder drum to rotate and thereby drive a shaft which is connected indirectly or directly to the cylinder drum.
As previously noted, axial-piston machines are available with inclined disks or swash plates or with drive flanges, i.e. systems in which the pistons react against but slide relative to an inclined surface and systems in which the piston hands entrained in rotation by the drum or which entrain the drum in rotation, depending upon whether the machine is operated as a motor or a pump, rotate with inclined surface.
To adjust the output of the machine, i.e. the volume of displaced fluid per revolution of the pump or the torque and speed of a motor, means can be provided for adjusting the angle included between the axis of the drum and the reaction surface for the pistons.
In drive-flange type axial-piston machines, the stroke volume per revolution is adjusted generally by one of two systems. In the first system, hereinafter denominated the pivotal-housing configuration, the cylinder drum is rotatable in the nonrotatable but swingable housing which can be tilted about an axis perpendicular to the axis of rotation of the drum to vary the inclination of its axis to the reaction surface. In the other system, hereinafter referred to as the swingable slide configuration, the cylinder drum within a fixed housing bears against a slide which is swingable to carry the drum into different positions and allow the inclination of its axis relative to the reaction surface to be adjusted. The swingable slide normally is guided along a curved guide surface in the fixed housing and has a complementary curvature. The invention is concerned primarily with an axial-piston machine of this latter type.
In swingable-slide, drive-flange axial-piston machines, the end of the cylinder drum toward the slide rotates upon a slide surface, in prior-art systems, which completely covers the end of the drum and hence has a cross section which is at least equal to that of the drum at the end thereof opposite that from which the pistons emerge. This surface of the control slide is usually provided with a pair of arcuate grooves (kidney-shaped grooves) separated from each other by webs and successively registering with the cylinder bores of the drum. One of these grooves communicates with the low-pressure side of the hydraulic system while the other groove communicates with the high-pressure side thereof.
To permit such communication, the side of the slide turned toward the housing is provided with two openings each communicating within the slide with a respective one of the arcuate grooves. A pair of passages are formed in the guide surface of the housing or otherwise therein for communication with the two openings of the slide. Each of these passages thus communicates with respective one of the openings and hence with a respective one of the arcuate grooves. These passages are generally elongated to permit the openings to communicate with them in all positions of the slide so that only a portion of each of the passages or channels communicates with an opening at any time while the slide covers the remainder of each channel. The two channels are connected via respective bores to the intake and discharge ports for the hydraulic medium.
An axial-piston pump of this general type is already known in which the suction side of the device communicates with the interior of the housing. In this case, the suction opening in the swingable slide communicates with a channel in the housing which is longer than the slide so that in all positions of the latter at least an end of the channel is no longer covered thereby and this end of the channel can communicate directly with the housing chamber surrounding the drum.
This system has been found to have the advantage that the intake of the pump includes the housing chamber and hence the hydraulic medium lubricates all of the moving parts while minimizing constrictions at the intake side and affording a cooling of the unit.
In addition, the arrangement has been found to reduce the noise output of the machine and to permit leakage oil to pass directly into the housing chamber and hence be returned to the pump cycle. Reference may be had, in this connection, to German Utility Model DT-Gbm No. 74 00 915.
German Printed Application DT-AS No. 1 653 417 describes a swash-plate machine, i.e. a machine in which the axis of the cylinder drum is fixed and the reaction surface for the pistons is tilted, in which there is provided between the housing and the cylinder drum a thin planar control disk which is provided with arcuate grooves for communication with the cylinders. At the suction side, this disk is interrupted along its outer periphery to permit communication between the intake side of the device and the housing.
However, this interruption of the control disk constitutes a disadvantageous weakening thereof which detrimentally affects the running of the pump. This arrangment has been found to be completely impractical for applications to drive-flange machines having swingable-slide configurations.
The system described in the aforementioned Utility Model also has been found to be unsatisfactory although it indeed eliminates the need for an additional passage or channel communicating between the intake groove and the interior of the housing.
In this arrangement, the flow resistance created by the prolonged channel and the arcuate groove which communicates therewith and with the cylinder drum has a high flow resistance which defeats, in large measure, the advantages of drawing fluid from the machine housing.